Thinning cirrus clouds to counteract climate change?: Cirrus cloud thinning (CCT) is a climate intervention method aimed at offsetting the anthropogenic greenhouse effect. Unlike stratospheric aerosol injections or marine cloud brightening, which aim to reflect more sunlight back to space, it is designed to increase the emission of longwave radiation to space. Cirrus clouds are made optically thinner by injecting appropriate ice nucleation particles, so that more cirrus clouds form by heterogeneous nucleation. The existing literature on the effectiveness of CCT is inconclusive, ranging from no effect to the equivalent of halving CO2 concentrations in an extreme emissions scenario.